Solid state lasers are widely used for marking, for cutting and welding steel, ceramics, and so forth, for medical laser scalpels, and so on. The application of these lasers as light sources for recording medium pick-ups has also been studied in recent years.
YAG-based materials (and especially Nd:YAG in which neodymium has been doped as an element that contributes to light emission (laser active element)) are routinely used as the medium for these solid state lasers. These materials are used in the form of single crystals manufactured by the Czochralski method (CZ method).
Methods have been proposed in which an Nd:YAG single crystal or Yb:YAG single crystal is joined with a YAG single crystal in order to achieve higher output operation than with a laser element made from an ordinary Nd:YAG single crystal (see U.S. Pat. Nos. 5,441,803, 5,852,622, and 5,846,638, for example).
In general, when the above-mentioned single crystals are joined together, this joining is carried out by cutting to a suitable size a YAG single crystal and an Nd:YAG single crystal grown by CZ method, then optically polishing the contacting faces, and heating the contacting faces (at a temperature that is 40 to 90% of the melting point, for example) under pressurization.
The above method, however, does not produce perfect matching in terms of crystal orientation between the Nd:YAG single crystals and YAG single crystals. Even if the joining faces are polished to a high precision, they will still be far from an ideal smooth surface, so it is difficult to form joining faces that are satisfactory as optical materials in a subsequent joining treatment (such as a hot pressing step).
Also, because the material strength of a YAG single crystal does not decrease very much at high temperatures, the material undergoes almost no deformation at the pressures involved in hot pressing (from a few dozen to a few hundred kilograms per square centimeter). Consequently, either just those portions of the two highly polished materials that happen to be smooth and come into contact end up being locally bonded, or gaps remain at the bonding interface according to the polishing precision.
Furthermore, with hot pressing it is impossible to manufacture a clad core type of composite laser element in which a YAG single crystal is joined to the peripheral portion of a cylindrical Nd:YAG single crystal, and particularly such an element having a structure in which the curved faces are joined. Specifically, with a clad core type, all that can be formed is a pseudo-type in which the core shape is square, rectangular, hexagonal, or the like. Furthermore, standard hot pressing involves uniaxial pressing in which a relatively high pressure is applied, and strain remains in the sample after treatment, so there are critical defects in the optical characteristics of the material that is obtained.
Meanwhile, a technique has been proposed in which the single crystals are joined together by subjecting the polished faces of a Ti:sapphire system to acid treatment and then heating to 1100° C. (see A. Sugiyama et al., “Direct bonding of Ti:sapphire laser crystal,” Applied Optics, 37 (12), pp. 2047 to 10, 1998).
In case of a bonded material such as the above, however, light scattering is observed at the bonding interfaces when the interfaces are irradiated with a He—Ne laser, and in a destructive test in which a mechanical impact is applied to the bonding, a large portion of the fractured face is fractured via a smooth face (the polished face prior to joining). It can be concluded from this situation as well that the above-mentioned technique does not adequately bond the single crystals together.
When laser oscillation is performed using an element that has not been adequately bonded, during output operation at a high photon density, the operational characteristics are only a fraction of what was intended in the composite laser element design. There are also cases in which beam quality decreases or there is a dramatic drop in the service life of the element, or when the laser element is broken near the bonding face.
In particular, a tremendous amount of energy is applied during excitation with a solid state laser, but even with semiconductor laser excitation, the excitation energy drops to about half in the interior of the medium, while over 90% of the energy becomes thermal energy in the case of lamp excitation (that is, it becomes energy that does not contribute to laser emission). Thermal energy generated inside the medium is transmitted to the outside by lattice vibration of the crystals, but because bonding is inadequate with a composite laser element produced by prior art, the lattice vibration does not transmit the energy well enough, so heat builds up at this point. As a result, the element deteriorates in stages at the joint in the case of weak excitation, and impact damage occurs in the case of strong excitation, and in both cases it is difficult to maintain the intended function of the composite element.
In contrast to these single crystal bonding techniques, a solid state laser oscillator has been proposed which makes use of a composite laser medium obtained by bonding together polycrystalline transparent ceramics (see Japanese Patent Application Publication 2002-57388). More specifically, a solid state laser oscillator has been proposed which has a polycrystalline ceramic composite laser medium obtained by optically polishing the contacting face of a polycrystalline transparent ceramic containing no active element and that of a polycrystalline transparent ceramic that has been doped with an active element, and then bonding these together. According to this method, it is stated that any heat generated in the polycrystalline transparent ceramic containing the active element can be effectively dispersed by the presence of the polycrystalline transparent ceramic containing no active element.
However, the above technique has a number of problems because the bonding method itself is premised on the same bonding method as a technique such as hot pressing for bonding single crystals.
Specifically, these problems include 1) strain is produced in the interior of the one or more types of polycrystalline transparent ceramic that are bonded, 2) chipping on the side faces or internal cracking occurs, and 3) bonding face misalignment occurs between the materials during pressing. Consequently, it is difficult to obtain a good bonding state, and in this respect it is difficult to obtain a polycrystalline composite laser medium that is high in optical quality.